1. Field of the Invention
The present invention relates to a torque sensor for utilizing a magnetostrictive effect anisotropy to detect a torque by a magnetic property change of a magnetostrictive film generated corresponding to the torque applied to a shaft, and a manufacturing method of the torque sensor.
Particularly, as an electromotive power steering apparatus for a car, the apparatus is constituted such that a rotation output of an electric motor as an auxiliary steering torque is reduced by a gear apparatus, and transmitted to an output shaft of a steering mechanism, a steering force applied to a steering wheel is assisted, and a wheel is steered. In the apparatus, a torque sensor for detecting the steering force transmitted to an input shaft, that is, the torque can preferably be used. The electric motor is driven in accordance with a detected result of the torque sensor, and a stable car driving can be realized by generating an auxiliary steering force.
2. Description of Related Art
In a magnetostrictive torque sensor, a magnetostrictive film such as an Nixe2x80x94Fe alloy film (foil) is formed on the surface of a shaft formed of a stainless steel or another nonmagnetic material, or SCM material or another magnetic material, and a torque is detected from a magnetic property change of the magnetostrictive film by a stress. In an operation principle of the sensor, a magnetostrictive effect anisotropy (magnetic elasticity effect anisotropy), that is, a phenomenon in which magnetic properties such as permeability or loss change by a magnetic anisotropy change caused by applying the torque to the magnetostrictive film is utilized. In further detail, the magnetic anisotropy change is converted to an inductance change, reactance change accompanying the inductance change, impedance change, resistance change in impedance, and the like by a magnetic circuit disposed outside, and the torque is detected by measuring the above change parameter.
For example, when a tensile stress is applied to the magnetostrictive film having a positive magnetostrictive constant, the permeability increases. Conversely, when a compression stress is applied to the film, the permeability decreases. When a permeability change of the magnetostrictive film by the stress is measured as an electric signal, the torque applied to the shaft can be detected.
The magnetostrictive torque sensor is broadly utilized as a torque sensor for detecting a torsional torque applied to a rotation shaft.
Moreover, for example, in data MAG-81-71 (1981) of Electrical Engineers of Japan (Magnetics), and Japanese Patent Publication No. 63876/1994, a manufacturing method of a magnetostrictive torque sensor is disclosed. The method comprises: applying a predetermined torsional torque to the rotational shaft in a predetermined direction; forming the magnetostrictive foil having magnetic anisotropy on the surface of the rotational shaft by adhesion bond; and subsequently removing the torsional torque, so that adverse influences such as an offset voltage in the vicinity of zero of the torsional torque and a difference in sensitivity to the torsional torque on left and right sides in the vicinity of zero of the torsional torque are removed. However, when the sheet of magnetostrictive foil is bonded to the shaft, the magnetic properties attributed to winding start and end of a thin band become uneven, and a problem of drift of an output with rotation of the shaft occurs.
Japanese Patent Application Laid-Open Nos. 184323/1987 and 164931/1984 disclose torque sensors in which the magnetostrictive film is directly formed on the shaft by a spray coating process, and a plating process, respectively. These torque sensors solve the problem that the magnetic properties become uneven with the bonded magnetostrictive foil and the problem that the output drift attributed to the uneven magnetic properties occurs. Moreover, in the torque sensor a uniform magnetostrictive film can be formed with respect to the shaft rotation, and a detecting coil is disposed/inclined to an axial direction so that the torque is detected. However, such a complicated detecting coil is required, and additionally the anisotropy preferable for detecting the shaft torque cannot be imparted in an oblique direction of 45 degrees. Moreover, in a journal of the Magnetics Society of Japan, vol. 22, p. 1074, another torque sensor is disclosed in which an NiFe sputtered film is formed on the shaft. Since the film is formed by a sputtering process, it is unnecessary to bond a magnetostrictive film prepared in another step to the shaft. Moreover, since the film is formed on the shaft being rotated, uniformity in a shaft circumferential direction is enhanced. However, it is also described that there are many restrictions on selection of the magnetostrictive material because of a relation between an inner stress and magnetostrictive constant during formation (deposition), and that a pattern shape is restricted.
Japanese Patent Application Laid-Open No. 185713/1998 discloses a strain sensor in which a current is passed through a strain raising material and a heat treatment is performed in a furnace. This solves a problem that anisotropy is incompletely imparted to the magnetostrictive film directly formed on the shaft by a plating process. In this method, an inductive magnetic anisotropy is applied in a right-angle direction to a shaft longitudinal direction, and the anisotropy preferable for detecting the shaft torque of an inclined 45 degree direction cannot be imparted.
In the aforementioned magnetostrictive torque sensor, the property is enhanced by a step of forming or bonding the magnetostrictive film onto the shaft with the torsional torque applied thereto and imparting the magnetic anisotropy in a manufacturing process.
However, when the magnetostrictive foil is bonded to the shaft with the torque applied thereto, a dispersion of a detecting property is disadvantageously generated. Particularly, in a method of forming only the magnetostrictive foil by a high-speed quenching process, and the like, and attaching the foil to the shaft, the bonding step is further added to the magnetostrictive foil forming step. Furthermore, when the foil is attached to the shaft, a stress is generated in the foil. Since the flat magnetostrictive foil sheet is attached along a circumference of the shaft, an attached portion is made in at least one portion of a circumference. Since the property of this portion is largely different from that of another portions, a dispersion is generated in a torque output value.
Moreover, when the magnetostrictive film is formed on the shaft with the torque applied thereto, a film forming chamber apparatus is generally small. Therefore, when the torque is applied to the shaft during the formation of the magnetostrictive film, a large restriction is imposed onto the apparatus. A production quantity per unit time decreases, and a cost increase results. Moreover, the magnetostrictive film is formed even on a jig for applying the torque, a countermeasure for preventing this problem is necessary, and an economical loss caused by formation of an expensive magnetostrictive film onto an unnecessary portion cannot be ignored. Furthermore, it is remarkably difficult to fix a middle portion of the shaft, and apply the torsional torque to the shaft in a complicated manner in which opposite ends of the shaft are twisted in the same direction during formation of the film.
As described above, various advantages are obtained when the magnetostrictive film is formed by the plating process, spray coating process, sputtering process or evaporation process. However, there is a large disadvantage that there is no process for applying a bias torque suitable for mass production.
The present invention has been developed to solve the aforementioned related-art problem, and an object thereof is to provide a magnetostrictive torque sensor and a manufacturing method of the sensor which can solve problems such as an insufficient productivity accompanying a restriction on the apparatus in a step of forming a magnetostrictive film to a shaft with a torque applied thereto, an economical loss caused by an unnecessarily formed film on a jig or another portion, a dispersion of a detecting property attributed to a step of bonding the magnetostrictive film to the shaft with a torque applied thereto, and a property dispersion in a shaft circumferential direction.
To attain the aforementioned object, according to the present invention, there is provided a torque sensor having: a shaft whose torque is to be detected; a magnetostrictive film formed on the shaft; and a coil for detecting a change of a magnetic property of the magnetostrictive film, wherein an intermediate film having a melting point lower than a melting point of the shaft and lower than a melting point of the magnetostrictive film is formed between the shaft and the magnetostrictive film.
Moreover, according to the present invention, there provided a torque sensor having a magnetostrictive film formed on a shaft whose torque is to be detected, wherein an intermediate film having a melting point lower than a melting point of the shaft and lower than a melting point of the magnetostrictive film is formed in a portion between the shaft and the magnetostrictive film.
Furthermore, in a preferred mode for carrying out the present invention, the intermediate film is formed of any metal selected from the group consisting of Sn, Pb, Bi, In, and Cd, or an alloy containing the metal selected from the group consisting of Sn, Pb, Bi, In, and Cd as a main component.
Additionally, in another preferred mode of the present invention, the intermediate film is formed by a plating process, spray coating process, sputtering process, or evaporation process.
Moreover, in further preferred mode of the present invention, the magnetostrictive film is formed by the plating process, spray coating process, sputtering process, or evaporation process.
Furthermore, in still further preferred mode of the present invention, the magnetostrictive film is constituted in a state in which a torque as a bias is applied to the torque to be detected.
Additionally, according to the present invention, there is provided a manufacturing method of a torque sensor having a magnetostrictive film formed on a shaft whose torque is to be detected, comprising the steps of: forming an intermediate film having a melting point lower than a melting point of said shaft, forming said magnetostrictive film having a melting point higher than the melting point of said intermediate film; subjecting said shaft with the torque applied thereto to a heat treatment at a temperature which is not higher than the melting point of said shaft, not higher than the melting point of said magnetostrictive film and not lower than the melting point of said intermediate film; and removing said torque after the temperature drops.
Moreover, according to the present invention, there is provided a manufacturing method of a torque sensor having a magnetostrictive film formed on a shaft whose torque is to be detected, comprising the steps of: forming an intermediate film having a melting point lower than a melting point of said shaft in a portion, forming said magnetostrictive film having a melting point higher than the melting point of said intermediate film; subjecting said shaft with the torque applied thereto to a heat treatment at a temperature which is not higher than the melting point of said shaft, not higher than the melting point of said magnetostrictive film and not lower than the melting point of said intermediate film; and removing said torque after the temperature drops.
According to the present invention, there can be a magnetostrictive torque sensor superior in uniformity of a shaft in a circumferential direction, and a manufacturing method of the sensor, which is simple, superior in productivity, and little in manufacturing dispersion. Moreover, a defective product produced in a manufacturing process is corrected, and a loss can be reduced.